This invention relates to a high voltage phasing detector, namely an instrument to determine whether the voltages on two conductors are in phase.
Such instruments are used, for example, to check phasing across switchgear and bus-bars after maintenance. The instruments comprise two resistors contained in insulating rods: one end of each rod has a contact probe, for contact with the conductors to be tested. The other ends of the rods are connected together via an insulated conductor and a detector, either in the form of an analog meter or an indicator such as a neon or light emitting diode. If the two conductors which the two probes contact are in phase, then no current flows and the detector reads zero (if it is an analog meter), or does not illuminate (if it is a neon or LED). However, if the two conductors are out of phase, the detector will give a reading (if it is an analog meter) or it will illuminate (if it is a neon or LED).
Under new standards to be introduced, a current of only 0.75 mA is available when the two conductors under test are out of phase. This current is sufficient to illuminate neons, but generally neons give poor illumination and are not particularly reliable. Light emitting diodes give good illumination, but require a relatively high current, typically 10 mA. Neither neons nor LED's have a precise switching point. It is possible to include a battery-driven arrangement in the instrument to provide a precise switching point, but this has the disadvantage that the battery must be replaced when discharged and further a circuit is required to detect when the battery is discharged.
We have now devised a high voltage phasing detector which overcomes these difficulties.